Method for injection stretch blow molding of polyethylene

Abstract: A novel method enabling stretch blow molding in a temperature range preferable for polyethylene, capable of releasing at a surface temperature suited to both releasing of a preform and subsequent stretch blow molding, by disconnecting the core die and the preform in advance by the use of pressure of gas. This is for injection molding a predetermined preform by filling an injection mold with molten polyethylene, releasing the preform from the cavity die and core die of the injection mold while a mouth portion of the preform is held by the lip die, and transferring the preform into a blow die to stretch blow mold into a hollow molded product of thin wall. Gas is forcedly entered into the boundary of the core die and the preform, before releasing the preform from the injection mold, to isolate the inside wall of the preform from the core die. Releasing is effected while the inside of the preform is not completely cooled and is still at high temperature, and in a temperature range in which the surface temperature of the preform right after releasing is 80 to 90 deg. C. The step of stretch blow molding is performed within the time interval before the surface temperature of the preform which is elevated by the internal heat of the preform reaches 120 deg. C.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for injection stretch blow molding of polyethylene into a hollow molded product, such as a thin-wall container. 2. Background Art In an injection stretch blow molding method comprising injecting molten resin into an injection mold to mold a preform, and stretch blow molding the perform into a hollow molded product such as container in a blow mold, it is known possible to mold almost all thermoplastic resins into hollow thin-wall products. At the present, however, being limited to polyethylene terephthalate, polypropylene, polycarbonate, vinyl chloride, etc., blow molding method is applied to polyethylene which is highly demanded as hollow molded products. Known methods for molding continuously from injection molding of preform to stretch blow molding of hollow molded products include a method comprising releasing an injection molded preform from an injection mold while the preform is still hot before being completely cooled and solidified, conditioning the temperature of the hot preform, and stretch blow molding the conditioned preform into a hollow molded product in a blow mold, and a method of immediately stretch blow molding by omitting the temperature conditioning of the preform as disclosed in the Japanese Laid-open Patent Publication No. 4-214322. In both injection stretch blow molding methods, a preform is molded by using an injection mold composed of a cavity die for molding the outside wall of the preform and a core die for molding the inside wall of the preform, and a lip mold for molding the mouth portion of the preform, and filling a cavity space between the cavity die and the core die inserted into the cavity die by penetrating through the opening of the cavity die and the closed lip mold, with molten resin by injecting the molten resin from the bottom of the cavity die. To release the injection molded preform from the injection mold, both the core die and lip mold are moved upwardly, or the core die is moved upwardly and the cavity die is moved downwardly, and the preform after being released is transferred into the blow mold while the mouth portion of the preform is held by the lip mold. The reason why the preform is released by being drawn out also from the core die is that it is extremely difficult to stretch the preform in the axial direction by the stretching rod in the state of tightly holding the core die by shrinkage of the preform due to cooling, and therefore the preform is drawn out from both the cavity die and the core die by using the lip mold also as the transfer member, thereby transferring into the blow mold. Usually, when releasing the injection molded product from the injection mold, withdrawing of injection molded product from the core die is more difficult than withdrawing from the cavity die. This is because the surface of the outside wall of the molded product is parted from the surface of the cavity die due to shrinkage by cooling of the injection molded product at the cavity side, while the inside wall of the molded product tightly contacts with the core die due to shrinkage, contrarily, to the core die side. The strength of the hot preform in a flexible state that can be processed by stretch blow molding is only enough to maintain the shape of the preform by the surface skin layer formed on the inside and outside surfaces of the preform, different from the ordinary injection molded product having rigidity on the whole by completely cooling and solidifying, and therefore also when drawing out the core die by holding the cooled and solidified mouth portion of the preform by the lip mold, unless there is enough strength required for disconnecting the skin layer of the inside wall of the preform in tightly contact with the core die surface, the preform is drawn out from the cavity die as being in tight contact with the core die, and squeezed by the lip die in fixed state, deforming as if bellows were contracted, thereby losing the shape of the preform. Accordingly, in an injection stretch blow molding method, it is designed to release the preform after cooling the preform to a temperature so that the skin layer of the preform surface is rigid enough to withstand the withdrawing force of the core die while stretch blow molding is possible. Although the cooling temperature varies with the thickness and design of the required preform, in the case of a preform of a container with a wide mouth of which blow-up ratio is not so large, the draft of the cavity die and core die can be set large, and the contact force of the preform inside by shrinkage can be alleviated by the draft, and releasing at high temperature is enabled, and the shrinkage due to cooling decreases, and owing to synergistic action of them, releasing is easier than in the case of a preform of a container with a narrow mouth. However, in the case of a preform of a container with a narrow mouth such as a bottle of which aperture is small, size is long, and blow-up ratio is required to be large, the draft of the cavity die and core die is limited, and the limit is stricter as the length is greater. It is hence necessary to release the preform from the injection mold by cooling the preform to the compatible temperature enabling both releasing and subsequent stretch blow molding. This compatible temperature is, at ordinary temperature, 60 to 70 deg. C in the case of polyethylene terephthalate, and 90 to 100 deg. C in the case of polypropylene, and in these temperature ranges, both releasing and stretch blow molding can be carried out. However, in the case of a preform of polyethylene which is excellent in thermal conductivity and high in molding shrinkage as compared with polyethylene terephthalate and polypropylene, when cooled to drawable temperature, the skin layer is formed thicker than necessary, and the internal high temperature region becomes narrow, and therefore if the preform is immediately transferred into the blow mold and processed by stretch blow molding, it is not swollen sufficiently by air pressure. At a temperature at which stretch blow molding is estimated to be possible, the preform remains tightly in contact with the core die, and when the core die is drawn out in this state, the preform is extremely deformed. Therefore, in the preform of polyethylene, setting of the compatible temperature enabling both releasing and subsequent stretch blow molding is more difficult than in the case of polyethylene terephthalate or the like, and stretch blow molding by using the hitherto employed releasing means was extremely difficult even in the case of a container with a wide mouth. It may be also considered to perform stretch blow molding by reheating and conditioning the preform after releasing to temperature suited to molding, but it requires experience, time, and energy, and temperature unevenness is likely to occur, and therefore if temperature conditioning is employed, the injection stretch blow molding of polyethylene involves many technical difficulties. Concerning polyethylene, aside from releasing, the difficulty also lies in the temperature for stretch blow molding the preform. In a blow molding method, the resin temperature for blow molding polyethylene is 175 to 200 deg. C. In an injection stretch blow molding method, such resin temperature is the molding temperature of the preform, and the temperature held by the preform cooled so as to be self-supporting is extremely lower than such resin temperature. When stretch blow molding the preform released at high temperature into a container such as a bottle before the surface temperature of the preform risen by the internal heat of itself reaches the peak temperature, the time until the surface temperature of polyethylene reaches the peak temperature is earlier than in the case of polyethylene terephthalate, and stretch blow molding polyethylene is difficult as compared with polyethylene terephthalate, and it is hard to obtain a molded product excellent in wall thickness distribution near the peak temperature. As the difficulty of stretch blow molding, it is estimated, since the thermal conductivity of polyethylene is higher than those of polyethylene terephthalate and polypropylene, transferring the internal heat of the preform to the surface of the preform after being released is relatively quick, the volume occupied by the internal high temperature portions contributing to the stretch blow molding becomes narrow earlier, and hence the internal heat quantity necessary for stretch blow molding tends to be insufficient. Hence, even in the case of container with a wide mouth easier to release than the container with a narrow mouth, concerning the stretch blow molding temperature, favorable molded product can not be obtained unless stretch blow molding is done within a limited time (within temperature range). Reaching of peak temperature differs somewhat depending on the wall thickness of preform, design or molding conditions, and better products are not obtained unless stretch blow molding is done accordingly, and therefore in the case of polyethylene, a severer molding operation is required than in the case of polyethylene terephthalate. The present invention is devised in order to solve the problems in a method for injection stretch blow molding of polyethylene, and it is an object thereof to present a novel method enabling to release a preform at a surface temperature of the preform compatible for both releasing of preform and subsequent stretch blow molding, by disconnecting the core die and the surface fo the preform in advance by the use of pressure of a gas, and to perform stretch blow molding in a specified temperature range preferable for polyethylene. The EP-A-0012426 discloses a method of injection stretch blow molding of organic plastic parison in which injection molding of a predetermined preform is conducted by filling an injection mold composed of a cavity die, a core die and a lip mold with molten resin, forcedly entering a gas into the boundary between the core die and the preform, releasing the preform from the cavity die and the core die of the injection mold while a mouth portion of the preform is held the by lip mold. Thereafter the preform is transferred into a blow die, and stretch blow molding the preform into a hollow thin-wall product is performed. The process disclosed in this document therefore is a method for temperature conditioning an organic plastic parison by retaining it in a mold cavity so as that a substantial change in the heat content of said parson has taken place. After that, the core is separated from said resin by applying positive fluid pressure between the core and the parson. The process comprises furthermore a temperature conditioning station for equalizing the temperature distribution of the resin. SUMMARY OF THE INVENTION To achieve the object, the invention presents a method of injection stretch blow molding, said method comprising the steps of; injection molding a predetermined preform by filling an injection mold composed of a cavity die, a core die and a lip mold with molten resin, forcedly entering a gas into the boundary between the core die and the preform, releasing the preform from the cavity die and the core die of the injection mold while a mouth portion of the preform is held by the lip mold, transferring the preform into a blow die, and stretch blow molding the preform into a hollow thin-wall product, wherein, said resin is polyethylene, the forcedly entering of the gas is carried out in order to isolate the inside wall of the preform from the core die and to cool the preform to a temperature suited to releasing, the releasing of the preform is carried out while the inside of the preform is not completely cooled and is still at high temperature, and the surface temperature of the preform right after releasing is 80° to 90° C, and the stretch blow molding of the preform is carried out within the time interval before the surface temperature of the preform which is elevated by the internal heat of the preform reaches 120° C releasing the preform from the injection mold while the inside of the preform is not completely cooled and is still at high temperature, and the surface temperature of the preform right after releasing is 80° to 90° C, and the stretch blow molding of the preform is carried out within the time interval before the surface temperature of the preform which is elevated by the internal heat of the preform reaches 120 deg.C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The temperature for the molten polyethylene at the time of molding the preform ist required to be 200 deg. C or more as the temperature of the front portion of the injection cylinder. This molten polyethylene is injected to fill up the cavity of the injection mold composed of the cavity die set at a temperature of 90 to 105 deg. C, and the core die set at a temperature around 80 deg. C, and molded into a preform with a bottom as prescribed. When the injection filling of molten polyethylene is completed, the injection pressure is changed to a secondary pressure of lower pressure than the primary pressure required for filling to transfer to injection pressure holding step, which is followed by cooling step. As to the gas for disconnection, air is most preferable from the viewpoint of economy and handling, but inert gas such as nitrogen gas may be used as the case may be. Meanwhile, if pressure feeding (i.e. entering gas) is started right after completion of injection filling, it gives no effect on the shape of the preform, but it is most preferable to start pressure feeding immediately after completion of injection pressure holding. If pressure feeding is started just before completion of injection filling, filling resistance occurs due to elevation of internal pressure by the pressure of the fed gas, and specified amount of molten resin is not applied, and hence the predetermined shape of the preform can not be obtained. To enter the gas, the gas is blown into the boundary of the core die and the preform from the root or front end of the core die. The entering position can be selected by the length or thickness of the preform, and in a long preform, it is desired to enter the gas from the front end. Besides, since the object of entering gas is to disconnect between the core die and the inside wall of the preform, the entered gas is held in the boundary for a specific time, and the preform inside is isolated from the core die surface, and the preform is cooled to a temperature suited to releasing, and the entering (blow) time requires at least one second, and the entering (blow) pressure is in a range of 6 to 15 kg/cm 2 , preferably amount 9 kg/cm 2 . The preform is released in the above-described temperature range, and the stretch blow molding can be effected in a temperature range of the preform surface temperature of 20 to 30 deg. elevated higher than that at releasing. If the surface temperature is 100 deg. C or less. the temperature is too low, and entire stretching by gas pressure is not achieved, and molding is extremely difficult. At elevation temperature of 30 deg. or more, it is too close to the peak temperature, and the temperature rise is slow, and the temperature state for the preform is likely to be crystallized. Accordingly, if the surface temperature of the preform after releasing exceeds 120 deg. C, stretch blow molding of molded product with favorable wall thickness distribution is difficult. Accordingly, the most preferable temperature for stretch blow molding is around a temperature elevated about 25 deg. from the surface temperature at the time of releasing, and it is in a range of 4 to 7 seconds after releasing in terms of the time. The peak temperature of the preform elevated by the internal heat is somewhat higher or lower depending on the wall thickness of the preform, but the time to reach the peak temperature do not vary significantly. Therefore, by varying the wall thickness distribution to adjust the internal heat, thereby causing elongation depending on the blow ratio on the preform, when containers of favorable wall thickness distribution and the like are obtained, except for the case of extreme wall thickness difference, stretch blow molding is enabled in a favorable temperature range on the whole within a specific time lapse after releasing. The air blowing pressure at the time of stretch blow molding is desired to be varied in primary and secondary steps, and the secondary pressure should be set at a pressure of two times or more of the primary pressure. Stretching can be done about two times vertically and three times laterally. In this method, when releasing the preform of polyethylene, gas is forcedly entered into the boundary between the core die and the inside wall of the preform, and cooling is effected with being disconnected the preform inside, and therefore cooling of the preform inside is suppressed by the gas existing in the boundary between the core die and the inside wall of the preform, and the inside skin layer of the preform is formed thinner than the case of tight contact of the inside skin layer of the preform, and the shrinkage of the preform inside is smaller by that portion, and the contact is alleviated. To the contrary, on the outside of the preform, the surface is pressed to the cavity side by the entered gas so as to be cooled easily, and separation by shrinkage due to cooling is prevented, and the skin layer is quickly formed so as to maintain the shape of the preform, and later it is possible to release the preform at a surface temperature suited to stretch blow molding. Because of quick cooling, moreover, crystallization is extremely small. Besides, in the process of elevation of surface temperature due to the internal heat of the preform, crystallization is suppressed by heating from inside, and its growth is slow, and stretching is not disturbed by crystallization, and stretch blow molding can be done smoothly even in polyethylene, and molded products such as containers of favorable wall thickness distribution can be obtained. [Embodiments] EXAMPLE 1 Material resin : Polyethylene (HIZEX 5300B, made by Mitsui Petrochemicals Co.) Molded product : Milk bottle (500 cc) Dimensions : overall height 165 mm, inside diameter of mouth portion 32 mm, length beneath neck 147.5 mm, outside diameter of body portion 73 mm, wall thickness of body portion 0.5 mm, weight 32 g Preform Dimensions : overall height 137.5 mm, inside diameter of mouth portion 32 mm, length beneath neck 120 mm, wall thickness of body portion 3 mm, outside dia. of upper body portion 34.68 mm, outside dia. of lower end body portion 31.62mm, draft 0.766 deg. Preform molding conditions Injection cylinder temperature : nozzle 175 deg. C, front portion 215 deg. C, middle portion 215 deg. C, rear portion 185 deg. C Injection mold temperature (set value): Cavity die : upper portion 10 deg. C, cavity portion 95 deg. C, lower portion 10 deg. C Core die : 80 deg. C Injection pressure (holding pressure) : 42 kg/cm 2 Pressure holding time : 6.5 sec Cooling time : 1.8 sec Entered (blow) gas : air Entering (blow) pressure : 9 kg/cm 2 Entering timing : right after completion of pressure holding Entering (blow) time : 1.8 sec Releasing temperature : 80 to 90 deg. C (preform surface temperature) Stretch blow molding conditions Mold temperature (set temperature) : 60 deg. C Stretch blow temperature : 105 to 115 deg. C (preform surface temperature) Blow pressure (stretching) : primary pressure 4 to 5 kg/cm 2 secondary pressure 12 kg/cm 2 Time interval from releasing till stretch blow molding : 6 sec. Stretching factor : vertical (axial) 1.2 times, lateral (radial) 2.2 times Results A milky white milk bottle of polyethylene uniform in wall thickness distribution without deviation of wall thickness in stretch area was obtained. When filled with content and dropped from a height of 2 m, no breakage was noted. EXAMPLE 2 Material resin : Polyethylene (HIZEX 5100B, made by Mitsui Petrochemicals Co.) Molded product : Round straight bottle with narrow mouth (120 cc) Dimensions : overall height 126.7 mm, length beneath neck 114.7 mm, inside diameter of mouth portion 17.14 mm, outside diameter of body portion 45.5 mm, wall thickness of body portion 0.5 mm, weight 15.4 g Preform Dimensions : overall height 107.0 mm, inside diameter of mouth portion 17.14 mm length beneath neck 95 mm, wall thickness of body portion 3.2 mm, outside dia. of upper body portion 22.03 mm, outside dia. of lower end body portion 18.71 mm, draft 1.0 deg. Preform molding conditions Injection cylinder temperature : nozzle 175 deg. C, front portion 210 deg. C, middle portion 210 deg. C, rear portion 195 deg. C Injection mold temperature (set value) : Cavity die : upper portion 10 deg. C, cavity portion 102 deg. C, lower portion 10 deg. C Core die : 80 deg. C Injection pressure (holding pressure) : 40 kg/cm 2 Pressure holding time : 5.45 sec Cooling time : 4.25 sec Entered (blow) gas : air Entering (blow) pressure : 9 kg/cm 2 Entering timing : right after completion of pressure holding Entering (blow) time : 4.25 sec Releasing temperature : 80 to 90 deg. C (preform surface temperature) Stretch blow molding conditions Mold temperature (set temperature) : 60 deg. C Stretch blow temperature : 105 to 115 deg. C (preform surface temperature) Blow pressure (stretching) : primary pressure 5 kg/cm 2 , secondary pressure 12 kg/cm 2 Time interval from releasing till stretch blow molding : 6 sec. Stretching factor : vertical (axial) 1.16 times, lateral (radial) 2.2 times Results A milky white round straight bottle of polyethylene uniform in wall thickness distribution without deviation of wall thickness in stretch area was obtained. When filled with content and dropped from a height of 2 m, no breakage was noted. These embodiments were conducted by using the injection stretch blow molding machine model No. BS-0207 manufactured by A.K. TECHNICAL LABORATORY, INC. In addition, the injection stretch blow molding method conformed to the method disclosed in the Japanese Laid-open Patent Publication No. 4-214322. According to the present invention, molded products of polyethylene such as containers with thin-wall body which had been hitherto considered extremely difficult can be molded easily by stretch blow molding, and a multiplicity of molded products can be produced at one time. Besides, since it is possible to blow in a thin wall, it is more economical than blow molding method, and the drop strength is sufficient, and hence its value of industrial usage is immense.

Claims: A method of injection stretch blow molding, said method comprising the steps of: injection molding a predetermined preform by filling an injection mold composed of a cavity die, a core die and a lip mold with molten polyethylene, forcedly entering a gas into the boundary between the core die and the preform, releasing the preform from the cavity die and the core die of the injection mold while a mouth portion of the preform is held by the lip mold, transferring the preform into a blow die, and stretch blow molding the preform into a hollow thin-wall product, characterised in that the forcedly entering of the gas is carried out in order to isolate the inside wall of the preform from the core die and to cool the preform to a temperature suited to releasing, the releasing of the preform is carried out while the inside of the preform is not completely cooled and is still at high temperature, and the surface temperature of the preform right after releasing is 80° to 90° C, and the stretch blow molding of the preform is carried out within the time interval before the surface temperature of the preform which is elevated by the internal heat of the preform reaches 120° C. A method for injection stretch blow molding of polyethylene according to claim 1, further characterized in that the entering gas is started right after completion of injection filling of molten polyethylene.

Inventors: KODA HIDEAKI, NAKAJIMA HISASHI, KODA, HIDEAKI, NAKAJIMA, HISASHI
Applicants: A K TECH LAB INC, A.K. TECHNICAL LABORATORY, INC.,
Application year: 1994
Main classification: B29C49
IPC codes: B29C45, B29C49